1. Field of the Invention
The present invention, generally, relates to a method and a system for cancelling noise from noise-corrupted speech and, more particularly, to an improved method and system for rendering speech recognizable in a high noise environment, particularly where noise is distributed.
One glance into the cockpit of today's commercial airliner would give an idea of the hands-busy, eyes-busy environment that exists there, and this is more true of the cockpit in today's military aircraft. The military has solved their problem somewhat by the use of voice-actuated controls for many activities, such as located in the cockpit of a fighter aircraft, and this has been accomplished through the use of voice recognition systems.
It was realized early that, due to the relatively high noise in the cockpit of a fighter aircraft, some form of noise cancellation was required, and from that need, an adaptive filter noise cancellation technique was developed that has become a standard in the industry. More recently, that technique was tried in military helicopters, and it was found to be ineffective.
2. Description of the Prior Art
It is understandable that the presence of high levels of noise in an audio signal will produce a substantial reduction in the intelligibility of speech, and it has been found that the most advanced voice recognition equipment is seriously ineffective in recognizing the simplest words in the high noise levels encountered in the cockpit of today's tactical fighter aircraft. A technique that was proposed by Bernard Widrow et al. in 1975, known as Adaptive Noise Cancellation (or ANC), has been tested extensively at the Research Laboratory of Electronics at the Massachusetts Institute of Technology.
The Widrow technique is described in an article that is entitled "Adaptive Noise Cancelling: Principles and Applications", Proc. IEEE, Vol. 63, No. 12, December, 1975.
During the M.I.T. tests, some improvements were developed in the Widrow technique, such as placements for the two microphones in a fighter cockpit environment as being one inside the oxygen facemask of the pilot and the second microphone outside the facemask. The one microphone, called the "primary" microphone, is located to sense, or to detect, the voice of the pilot plus the noise.
The second, or "reference", microphone is located to sense, or detect, principally the noise. By locating the reference microphone outside the oxygen facemask, very little of the pilot's voice is picked up.
The engineers at M.I.T. learned also that it is better to have the signal-to-noise ratio of the primary microphone large compared to the signal-to-noise ratio of the reference microphone, so that the adaptive filter can be kept as small as possible. Otherwise, the adaptive filter must either estimate the delay between the primary and reference signals or have a long impulse response in order to provide good cancellation of the noise from the primary signal.
A report of the M.I.T. engineers is given in a paper entitled "Adaptive Noise Cancellation in a Fighter Cockpit Environment" by Harrison, Lim and Singer, 1984 IEEE, pages 18A.4.1 through 18A.4.4.
With all of the expertise of these M.I.T. engineers, the conclusion was that the Adaptive Noise Cancellation technique of Widrow, while effective enough in an environment with a localized noise source, degrades in performance when there is more than one noise source present or when the noise source is distributed over a region. Actually, the many sources of noise in a helicopter make the Adaptive Noise Cancellation technique virtually ineffective in that high noise environment where the noise sources are distributed over a wide region. While those experts in the field departed to study the use of additional reference microphones in a distributed noise environment, the present invention proceeds with the development of a unique solution to this perplexing problem.
A review of the prior patent art reveals very little to assist in developing a solution such as provided by the present invention. For example, U.S. Pat. No. 4,625,083 to Poikela is concerned with providing a voice operated switch that is capable of distinguishing between voice and noise. By using one microphone primarily for speech and one microphone primarily for ambient noise signals, each of these groups of signals have a certain sound pressure level, and since it is desired to have the sound pressure level of the speech signal always exceed that of the noise signal, this is accomplished in two ways. One way is by placing the two microphones in predetermined locations so that the sound pressure level distinctions are realized, and another way is by limiting the width of the frequencies, like that customarily used in telephone receivers. A typical frequency range is 100 hertz to 4 kilohertz, but a narrower frequency range of 250 hertz to 3.5 kilohertz is termed as being satisfactory. By connecting both signals to a differential amplifier, an output will result when there is speech, and there is no output when there is no speech.
U.S. Pat. No. 4,649,505 to Zinser, Jr. et al. is an example of another attempt to improve on the basic adaptive filter of Widrow, identified supra, but this effort is for the purpose of eliminating crosstalk between speech and noise signals. It discloses the use of a speech input, a noise input and a reference input with a reference noise portion and a crosstalk speech portion to a digital signal processing microcontroller, a read-only-memory and a random access memory, from which the signals are processed digitally. After the inputs are converted first from analog to digital signals, they are converted next from digital serial signals to digital parallel signals for further processing. There is no mention of the problem with which the present invention is concerned.
U.S. Pat. No. 4,658,426 to Chabries et al. discloses several different forms of noise suppression devices for use where the signal-to-noise ratio is poor at the input and where the characteristics of the adaptive filter adjust automatically to variations in the input signal. These adjustments utilize time and frequency domains in making the adaptive filter adjustments in order to filter noise, and a mathematical description is given in substantial detail for devices constructed to take advantage of such premises. A use for such devices is given as one tuned to filter out the normal operating sound of machinery as "noise" and to detect the unusual sound of a worn or failed component of the machinery. However, these are illustrations of localized noise, with which the adaptive filter type of device is capable of functioning quite adequately, according to the M.I.T. reference, supra.
U.S. Pat. No. 4,672,674 to Clough et al. discloses a system utilizing two specially built microphones that have good near field response and poor far field response to produce signals with noise components having high correlation. Like the Poikela U.S. Pat. No. 4,625,083 above, the outputs from these microphones are connected to a filter to remove frequencies outside the range of 300 Hz to between 5 and 8 kHz. The signals then pass to analog-to-digital converters, to micro-processor circuitry having delay and other capability, to achieve weighted-factor-samples for further processing. While this prior patent discloses the use of two microphones, it also suggests that a logical extension of this use is to use three or more microphones, one for speech and the outputs of the other microphones being used to cancel the noise in the signal from the one microphone.
On the other hand, the present invention takes a different approach to providing a solution to the problem of cancelling distributed noise from a speech signal, because tests show that the Adaptive Noise Cancellation technique of the prior art degrades in performance when the noise is distributed over a region.